Ion implantation is used in semiconductor wafer fabrication to add impurities (ions having certain electronic characteristics) to a substrate. One form of ion implantation apparatus utilizes an ion beam which is scanned over the surface of the wafer. The ions of the beam are produced with sufficient energy that they are implanted into the wafer material. The problem with this method of ion implantation is that the amount of time it takes to scan an ion beam over the surface of a wafer increases significantly as the wafer area increases.
A second method used for large wafer ion implantation is termed plasma immersion. In this method, a wafer is positioned within a plasma and is pulse biased to a highly negative voltage. The ions of the plasma bombard the wafer and are implanted. The problem associated with this method is that the ions of the plasma which bombard the wafer have a distribution of energies and as a result the ions which are implanted penetrate into the wafer to various depths. This results in a non-uniform implantation layer. Furthermore, this method has no control over the composition of the ion species implanted since all of the positive ion species in the plasma will be implanted. To avoid these undesirable effects, the energy distribution of the ions bombarding the substrate must be controlled and the composition of the ion species for implantation must be limited.